In his work The Orthodox Corruption of Scripture (New York: Oxford University Press, 1993) Bart Ehrman explains why he 'felt constrained' to leave the Comma Johanneum out of his book.
Although (Ehrman writes) the famed Comma "represents the most obvious instance of a theologically motivated corruption in the entire manuscript tradition of the New Testament," (page 45, note 116) its appearance in the MS tradition "can scarcely be dated prior to the trinitarian controversies that arose" subsequent to the period of time examined in his monograph. In other words, the Comma is a late and spurious interpolation that does not belong in Holy Writ.
Ehrman (page 45) then references Catholic scholar Raymond E. Brown's excellent delineation of this textual issue in his Anchor Bible Commentary on The Epistles of John. See Brown, pp. 775-87. I heartily recommend Brown's thorough analysis of the issues surrounding the Comma Johanneum. Brown also shows that the Comma is an interpolation.
Rudolph Bultmann (_N.T Epistles of John_ [Die drei Johannesbriefe]. Gottingen: Vandenhoeck & Ruprecht, 1969. Edition 8. Aufl.)--whatever you may think of his theology--seems to be on the mark (for the most part) vis-a-vis the textual tradition when he notes:
"The so-called Comma Johanneum (vss 7f) is found in Latin [MSS] dating before A.D. 400, the text of which(it varies in details in individual manuscripts) was also taken up into the Sixto-Clementine edition of the Vulgate in the following form: 'Quoniam tres sunt, qui testimonium dant in coelo Pater, Verbum et Spiritus Sanctus, et hi tres unum sunt.' ('For there are three who bear witness in heaven, the Father, the Word, and the Holy Spirit, and these three are one. And there are three who bear witness on earth, the Spirit and the water and the blood, and these three are one.') Although the decree of the Holy Office in 1897 decided in favor of the authenticity of the Comma Johanneum, today its spuriousness is also recognized by Catholic scholars. The passage is unknown to the entire Greek tradition" (Bultmann 81).
I am not sure about Bultmann's last observation in the quote above. Maybe he means that the JC does not appear in any of the early Greek MSS. For we know that it evidently does occur in the 14-15th century and it likewise appears in the 16th century. At any rate, it is more surely a late corruption of the Greek text as Ehrman
and Brown note: the Comma does not belong in Scripture. This is also the judgment of Westcott.